1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate for an in-plane switching mode liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have orientation characteristics of arrangement resulting from their thin and long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field to them. In other words, as the electrical field applied is changed, the alignment of the liquid crystal molecules also changes. Since the incident light is refracted according to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, images can be displayed.
Recently, active matrix LCD (AM-LCD) devices, which have thin film transistors and pixel electrodes arranged in a matrix form, have become the subject of significant research and development because of their high resolution and superiority in displaying moving images. The AM-LCD devices include upper and lower substrates and a liquid crystal layer disposed between the upper and lower substrates. The upper substrate, referred to as a color filter substrate, includes a common electrode. The lower substrate, referred to as an array substrate, includes a pixel electrode. As a result, AM-LCD devices drive the liquid crystal layer by varying multitude of electrical fields between the common and pixel electrodes and have excellent transmittance of light and aperture ratio.
However, a driving mode mentioned above has a disadvantage in bed sight angle. To resolve this disadvantage, a new driving mode, such as an in-plane switching mode, has been suggested. An LCD device illustrated below use the in-plane switching mode (IPS), so that they have excellent sight angle.
FIG. 1 is a cross-sectional view of an IPS-LCD device according to the related art. As shown in FIG. 1, the IPS-LCD device B includes a liquid crystal layer LC, upper and lower substrates B1 and B2. The upper and lower substrates B1 and B2 face each other, and the liquid crystal layer LC is interposed between the upper and lower substrates B1 and B2.
The lower substrate B2 includes a substrate 50, thin film transistor T, common electrode 58 and pixel electrode 72. The substrate 50 has pixel regions P1 and P2. The thin film transistor T, the common electrode 58 and the pixel electrode 72 are formed on each pixel region P1 and P2. The thin film transistor T includes a gate electrode 52, a semiconductor layer 62, a source electrode 64 and a drain electrode 66. The gate electrode 52 is formed on the substrate 50, and the semiconductor layer 62 is formed over the gate electrode 52 with a gate insulating layer 60 interposed therebetween. The source and drain electrodes 64 and 66, which are separated from each other, are formed on the semiconductor layer 62.
Generally, the common electrode 58 is made of the same material and on the same layer as the gate electrode 52. The pixel electrode 72 is made of the same material and on the same layer as the source and drain electrodes 64 and 66. However, as shown in FIG. 1, the pixel electrode 72 is made of a transparent material to improve the aperture ratio. A gate line, not shown, is formed along a side of the pixel regions P1 and P2, and a data line, not shown, is formed perpendicular with the gate line. The gate and data lines define the pixel regions P1 and P2. A common line, not shown, which applies a voltage to the common electrode 57, is also formed on the substrate 50.
The upper substrate B1 includes a substrate 30, a black matrix 32, and color filters 34a and 34b. The black matrix 32 is formed at portions corresponding to the thin film transistor T, the gate and data lines. The color filters 34a and 34b are formed at portions corresponding to the pixel regions P1 and P2. The IPS-LCD device is driven by horizontal electrical field 95 between the common and pixel electrodes 58 and 72.
FIG. 2 is a plane view of a lower substrate for an IPS-LCD device according to the related art. As shown in FIG. 2, the lower substrate includes a substrate 50, a gate line 51, a data line 61, a common line 70 and a thin film transistor T. The gate line 51 and the data line 61 cross each other to define a pixel region P. The common line 70 is separated from the gate line 51 and also parallel to the gate line 51. A thin film transistor T is formed at a crossing portion of the gate 51 and data lines 61, and includes a gate electrode 52, a semiconductor layer 62, source and drain electrodes 64 and 66. The gate electrode 52 is connected to the gate line 51. The semiconductor layer 62 is formed on the gate electrode 52. And the source and drain electrodes 64 and 66 are formed on the semiconductor layer 62.
A common electrode 58 and a pixel electrode 72 are formed on the pixel region P. The common electrode 58 is connected to the common line 70 and extends perpendicularly from the common line 70 to the pixel region P. The pixel electrode 72 is connected to the drain electrode 66 and is alternatively arranged with the common electrode 58. In the lower substrate having the above mentioned structure, there is a step difference between the common and pixel electrodes 58 and 72, since the common and pixel electrodes 58 and 72 are formed on different layers. Therefore, the electric field has non-uniform distribution.
FIG. 3 is a cross-sectional view taken along a line III-III of FIG. 2. As shown in FIG. 3, a substrate 50 has a pixel region P. First, a common electrode 58 is formed on the substrate 50 in the pixel region. The common electrode 58 is made of the same material as a gate line at the same time. A gate insulating layer GI is formed on the common electrode 58, and a data line, not shown, is formed on the gate insulating layer GI. In addition, a pixel electrode 72 is formed on the gate insulating layer GI in the pixel region P.
Because the common electrode 58 and the pixel electrode 72 are formed on the upper and lower surface of the gate insulating layer GI, and since the gate insulating layer GI is made of inorganic material, the gate insulating layer GI has a step difference. It is very difficult to obtain a relatively low step difference when the inorganic material layer is made by a deposition process.
Therefore, as shown in FIG. 3, the distribution of the electric field 95 between the common electrode 58 and the pixel electrode 72 becomes inclined to the left or right. The inclined distribution could result from misalignment caused in the process of forming the common electrode 58 and pixel electrode 72 on different layers. The inclined distribution becomes more severe when the pixel electrode 72 is made of a transparent material.
FIG. 4 is a cross-sectional view of array substrate for IPS-LCD device according to the related art. As shown FIG. 4, a common electrode 58 is formed on a substrate 50. The common electrode 58 is made of the same material and the same layer as a gate line. Then, a gate insulating layer GI is formed on the substrate 50 having the common electrode 58, and a data line, not shown, is formed on the gate insulating layer. A passivation layer PL is formed on the substrate 50 having the data, and a transparent pixel electrode 72 is formed on the passivation layer PL.
Since the gate insulating layer GI and passivation layer PL are interposed between the common electrode 58 and pixel electrode 72, the gate insulating layer GI and the passivation layer PL have step differences as high as a thickness of the common electrode. As mentioned above, the step differences are generated because the gate insulating layer GI and the passivation layer PL are made of inorganic material. The step differences affect the distribution of electric field such that the distribution of electric field between the common electrode 58 and pixel electrode 72 becomes inclined to the left or right. Accordingly, the LCD device using the substrate according to the related art has problems represented by residual images and non-uniform image quality.